Not sure if this has been posted here before or not, apologies if it has. Came across this very comprehensive report (from 2011) of all the major trunked systems in Southeast Michigan, and their interoperability with each other. Some pretty decent technical reading of the different systems, their strength's and weaknesses and suggestions on how they could be improved.
http://www.dccwf.org/main/pdf/RPT110505LRK_SEMI_UASI_Public_Safety_Gap_Analysis_Final.pdf
http://www.dccwf.org/main/pdf/RPT110505LRK_SEMI_UASI_Public_Safety_Gap_Analysis_Final.pdf